1. Field of the Invention
The present invention relates to a method for applying coatings, particularly color/clear composite coatings, to articles of manufacture for outdoor exposure such as motor vehicles. More particularly, the invention relates to an improved method for applying coatings to such articles in which the resultantly coated substrate is treated to improve its resistance to water spotting and acid etching.
2. Brief Description of the Prior Art
Color/clear composite coating systems involve the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear top coat to the base coat. These composite coatings are becoming increasingly popular as original finishes for motor vehicles. The color/clear finishes have outstanding gloss and distinctness of image and the clear coat is particularly important for these properties. A serious problem associated with this type of finish is water spotting or acid etching. Water spotting is the tendency of water to permanently spot the coating when placed in direct contact with the coating. Acid etching is the tendency of acidic materials to actually etch away the clear coating when in direct contact with the coating. Because many portions of the world, particularly industrial areas, have an acidic environment, rain water becomes acidic and when left on the painted surface of the motor vehicle will cause water spotting and, if the rain water is acidic enough, will actually acid etch away a portion of the clear coat. If serious enough, the motor vehicle may have to be repainted since it is very difficult and often impossible to polish and buff out the water spotting and acid etching. The problem is also present in color coatings without a clear coat, although it is not nearly as severe as with clear coats.
The problem of water spotting and acid etching has been recognized in the industry and one approach to solve the problem is to formulate clear coat compositions with specific resinous binders which are resistant to water spotting and acid etching. Examples of such resinous binders are those described in U.S. Pat. No. 4,818,796. Although formulating clear coating compositions with specific resinous binders so as to have improved resistance to water spotting and acid etching is a solution to the problem, there are certain drawbacks associated with this approach. For example, the compositions of U.S. Pat. No. 4,818,796 have a tendency to crack when exposed for long periods outdoors, particularly in hot, humid atmospheres. Also, new clear coat compositions must go through a time consuming approval process before they will be accepted by the industry.
Therefore, it would be desirable to take formulated coating compositions which are presently approved by the industry and treat the coated substrate in a manner so as to improve its resistance to water spotting and acid etching.